1. Field of the Invention
The present invention relates to an attachment for a heat shield, in particular one usable with the cylinder head of an internal combustion engine equipped with a valve to control the transfer of gases in the engine's combustion chamber.
2. Description of the Prior Art
In known types of combustion engines, the combustion chambers are defined in general by the inner walls of a cylinder, by the face of a piston moving in the cylinder under the combined effect of the pressure of the gases in the combustion chamber and the inertia of the piston's mechanical drive structure, and by the inner walls of a cylinder head closing the cylinder, the cylindrical head often constituting the hottest part of the combustion chamber's jacket.
Rotary combustion engines have a combustion chamber delimited by the outer surface of a rotary piston and by the walls of a cylinder head enveloping the rotary piston. In such rotary engines, combustion of the gaseous fuel mixture always occurs in the same area, with no gaseous cooling by the cool fuel gases taken into the combustion chamber.
In order to limit the wear and tear caused by superheating of the walls of the combustion chamber and, especially in diesel engines, in order to benefit from the effect of distillation and vaporization of the liquid fuel by superheated walls, it has already been proposed that some of the walls of the combustion chamber of thermal engines be covered by a heat shield to protect the wall's metal from superheating. In addition, because of its relative thermal insulation properties, the heat shield is locally superheated to vaporize and ignite the liquid fuel injected into the combustion chamber.
The most frequently used materials for these heat shields are ceramics consisting for example of zirconium oxides, silicon carbides or nitrides prepared by sintering after hot or cold pressing. Other materials used for these heat shields are refractory materials and/or alloys that are poor conductors and that have good resistance to thermal shocks so as to withstand superheating over a large surface that is uncovered with each combustion cycle.
Attaching these shields is difficult by reason both of their coefficient of expansion, which is often different from that of the material in the element to be protected, and because of the significant temperature gradients present in the combustion chamber. The anchorings usually provided for these attachments do not take into account the relative variations in sizes between the shield and the element, which generally results in cracking followed by destruction of the shield or the element to be protected. In addition, these anchorings must be sufficiently close together, as the shields have relatively thin walls and have a tendency to vibrate, which causes further mechanical stress.